SFPM Calculator
SFPM stands for surface feet per minute, the cutting speed at the edge of a spinning tool or workpiece. Every CNC speed chart and Machinery's Handbook entry spec's cutting speed in SFPM because the number stays the same across tool sizes, so tool life stays predictable. This SFPM calculator runs the standard SFPM = (pi x D x RPM) / 12 formula in either direction.
SFPM formula
- SFPM = (pi x D x RPM) / 12 with D in inches.
- Solved for RPM: RPM = (SFPM x 12) / (pi x D).
- Metric: Vc (m/min) = (pi x D_mm x RPM) / 1000.
For a 1/2 inch carbide end mill at 3000 RPM: SFPM = (pi x 0.5 x 3000) / 12 = 392.7 SFPM. To hit 400 SFPM on the same tool, RPM = (400 x 12) / (pi x 0.5) = 3,056 RPM. The SFPM calculator runs either direction.
Recommended SFPM by material
| Material | HSS SFPM | Carbide SFPM |
|---|---|---|
| Aluminum 6061 | 600 | 1400 |
| Mild steel (1018) | 100 | 400 |
| Alloy steel (4140) | 70 | 250 |
| Stainless steel | 60 | 200 |
| Cast iron | 80 | 260 |
| Brass | 250 | 500 |
| Titanium Ti-6Al-4V | 40 | 120 |
| Plastic (Delrin) | 800 | 2000 |
Why SFPM and not RPM directly
Cutting speed is a property of the material-tool pair, not the tool size. Mild steel with carbide wants 400 SFPM whether the tool is a 1/8 end mill, a 1 inch drill, or a 3 inch bar on a lathe. Only the RPM changes. SFPM lets you keep one cutting speed table and apply it to any tool diameter: just plug into RPM = (SFPM x 12) / (pi x D). An SFPM calculator is the translator.
Which diameter to use
The D in the SFPM formula is whichever surface is spinning:
- Milling: end mill or face mill cutting diameter.
- Drilling: drill diameter.
- Turning: workpiece OD currently being cut (not the tool, because the tool is stationary).
This is why a lathe SFPM calculation uses workpiece diameter, but a mill SFPM calculation uses tool diameter. Same formula, different D.
Calculating RPM from SFPM
Start with the SFPM target from a Machinery's Handbook table or a tool vendor data sheet. Plug into RPM = (SFPM x 12) / (pi x D). For a 3/8 inch carbide drill in mild steel at 250 SFPM: RPM = (250 x 12) / (pi x 0.375) = 2,546 RPM. If your machine tops out below that, cap at the machine max and accept the lower SFPM.
SFPM vs SFM
SFPM and SFM mean the same thing: surface feet per minute. Older texts and some vendors spell it out as SFPM; most modern tables shorten it to SFM. The number and the formula are identical. This SFPM calculator accepts either name and returns the same value.